What really happened?

At 9:51 am (Greenwich Mean Time) on September 14, 2015, something happened which will change the way we look at the Cosmos…

What really happened?

At 9:51am (Greenwich Mean Time) on September 14, 2015, something happened which will change the way we look at the Cosmos. A window opened in the sky which showed us an event which occurred more than one billion years ago, generating 50 times more luminosity (or light power) than all the stars and galaxies combined, as two black holes collided in the depths of outer space, to produce a single, even bigger spinning black hole.

Within a fraction of a second, the collision released nearly six million trillion trillion kilograms of energy, or three times the mass of the Sun. To put this in perspective, the Sun converts only two billionths of a trillionth of its mass into electromagnetic radiation every second, so work out for yourself how long it would take to generate the same amount of energy.

The only evidence of this cataclysmic event, however, was not a giant explosion but mirrors moving such a small distance that you'd need a giant magnifying glass to be able to see it – a millionth of a millionth of a hair’s width, lasting just a few hundred milliseconds. You could blink your eyes and miss it, but that tiny movement was one of the most important milestones in the history of science – the first evidence of a phenomenon first dreamed up in the mind of Albert Einstein 101 years ago, called gravitational waves.

Einstein may have done all his sums right, but even he did not expect the evidence would ever be found. But as Professor Sheila Rowan, Director of the Institute for Gravitational Research (IGR) at Glasgow University pointed out, Einstein would also have been surprised that General Relativity has played such an important role in so many modern devices, including GPS navigators, not even dreamed of in his time.

Even though the first detection of a gravitational wave may have seemed more like a whimper than a bang, it is only dramatic events such as colliding black holes or neutron stars, or massive stars collapsing, that make the mirrors move enough for us to detect any movement at all. Any object which accelerates produces gravitational waves, including human beings, but these are too small to detect. In fact, it isn’t even remotely possible to build a machine that can spin an object fast enough to produce a detectable gravitational wave – even the world’s strongest materials would fly apart at the rotation speeds such a machine would require.

Since we can’t generate detectable gravitational waves on Earth, the only way to study them is to search the distant Universe for evidence of the incredibly massive objects that undergo rapid accelerations – e.g., neutron stars and black holes.

Theory plus teamwork

To understand the types of gravitational waves that different objects may produce, LIGO scientists have defined various categories of gravitational wave events, each with a unique “fingerprint” or characteristic vibrational signature, and the first detection was a Compact Binary Inspiral Gravitational Wave, produced by the merger of two massive black holes.

Black holes have come together fairly frequently since time began, but we could not observe a collision or prove it had happened until a group of scientists – from countries all around the world, including Scotland – got together to develop a completely new kind of detector which is capable of “sensing” ripples in the fabric of space-time caused by gravitational waves.

The detectors may have come up with the data, but it was scientists around the world who built the tools: “To make this fantastic milestone possible took a global collaboration of scientists – laser and suspension technology developed for our UK/German GEO600 detector was used to help make Advanced LIGO the most sophisticated gravitational wave detector ever created,” said Professor Jim Hough, Associate Director of the Institute for Gravitational Research at Glasgow University.

The “cosmic detectives” used two detectors, thousands of miles apart (at Livingston, Louisiana and Hanford, Washington) in order to confirm that what they sensed was not a freak event which happened in only one place – independent and widely separated observatories are necessary to determine the direction of the event and verify that the signals are not a local phenomenon.

The detector in Livingston recorded the event seven milliseconds before the detector in Hanford, and this enabled scientists to identify the source in the Southern Hemisphere, using triangulation – Livingston is south of Hanford.

“Hopefully, this first observation will accelerate the construction of a global network of detectors to enable accurate source location in the era of multi-messenger astronomy,” said David McClelland, Professor of Physics and Director of the Centre for Gravitational Physics at the Australian National University.

The signal was identified as a candidate gravitational wave after an initial analysis of the data, three minutes after it arrived at the Earth, using a burst algorithm technique developed within the ‘burst’ working group at LIGO, now chaired by the IGR's Professor Siong Heng. Matched filtering was then applied, to compare the data to predicted signals (‘waveforms’) in order to find the best match. Further checks were carried out to ensure that no environmental or instrumental effects could have caused the signal – anything from misbehaving electronics to faraway lightning strikes – before the team concluded that the signal must have had an astrophysical origin.

A new type of science...

The first detection took a few weeks to confirm, but when the scientists announced their great discovery, excitement quickly spread around the world.

“Our observation of gravitational waves accomplishes an ambitious goal set out over five decades ago to directly detect this elusive phenomenon and better understand the Universe,” said Caltech’s David H. Reitze, Executive Director of the LIGO Laboratory.

After the first wave of excitement had passed, the data were compared with Einstein's theoretical predictions “to test whether general relativity is able to fully describe the event,” and passed the test “with flying colours.” In other words, Einstein was right all along and the detector had proved it.

To see black holes colliding and detect gravitational waves was a major achievement, but this is just the first step on a journey to develop a completely new field in science. The initial breakthrough was to gather information from the real world which confirms the theoretical predictions and helps us understand the strange behaviour of black holes before and after they collide and merge – e.g., the black hole was spinning, and this confirmed what mathematician Roy Kerr had predicted in 1963.

The discovery was not just the end of a project to prove Einstein's ground-breaking theory, but also marked the beginning of gravitational-wave astronomy as a revolutionary new means to explore the frontiers of our Universe: “This detection is the beginning of a new era. The field of gravitational-wave astronomy is now a reality,” said Gabriela González, Professor of Physics and Astronomy at Louisiana State University, who was the LSC spokesperson at the time.

Virgo spokesperson Fulvio Ricci added: “This is a significant milestone for physics but, more importantly, merely the start of many new and exciting astrophysical discoveries to come.”

“With this discovery, we humans are embarking on a marvellous new quest: the quest to explore the warped side of the Universe – objects and phenomena that are made from warped space-time. Colliding black holes and gravitational waves are our first beautiful examples,” said Kip Thorne, Caltech’s Richard P. Feynman Professor of Theoretical Physics.

Human beings first used their eyes to look up at the heavens, then later on invented telescopes and more advanced detectors able to “see” things not visible to the naked eye, including infrared and X-rays, etc. Nothing escapes from inside a black hole, not even gravitational waves, but the evidence provided by LIGO enables us to understand what is happening very close to the black holes, and also confirms that we now have a new way to study the Cosmos and see things far beyond our wildest dreams.

Mirror, mirror

LIGO (the Laser Interferometer Gravitational-wave Observatory) is the world’s largest gravitational-wave observatory and one of the world's most sophisticated physics experiments. The detectors have also been described as “the most sensitive scientific instruments ever constructed.”

There are two LIGO detectors, both located in the USA, thousands of kilometres apart, funded by the National Science Foundation (NSF), designed and run by Caltech (the California Institute of Technology) and MIT (the Massachusetts Institute of Technology). The LIGO Scientific Collaboration (LSC) is a group of more than 1,000 scientists from more than 90 universities and research institutes in 15 countries, including the GEO Collaboration (which involves Germany, the UK and Spain), and the Australian Consortium for Interferometric Gravitational Astronomy.

The LSC also works closely with the French–Italian Virgo Collaboration which, as well as having its own detector (currently being commissioned), uses data from the LIGO detectors.

The LIGO Laboratory is also working closely now with scientists at the Inter-University Centre for Astronomy and Astrophysics, the Raja Ramanna Centre for Advanced Technology and the Institute for Plasma Research, to establish a third Advanced LIGO detector in India, which could be operational early next decade and would greatly improve the ability of the global detector network to identify and locate the sources of gravitational waves. There is a further collaboration with the KAGRA project in Japan, which is building a detector in the Kamioka mine that should be operational within a few years.

LIGO uses the physical properties of light and of space itself to detect gravitational waves – a concept first proposed in the early 1960s. Each detector has two “arms”, each 4km long, positioned at right angles to each other. In the L-shaped interferometer, the laser light is split into two beams that travel back and forth down the arms (four-foot-diameter tubes kept under a near-perfect vacuum). The beams are used to monitor the distance between the mirrors – suspended on pendulums for seismic isolation – at the ends of the arms. When a gravitational wave passes by, stretching and squashing space, it lengthens and shortens the arms, changing the time it takes the laser beams to travel through the arms.

This means that the two beams are no longer “in step” and produces what is called an interference pattern – which is why the detectors are known as “interferometers”.

According to Einstein’s theory, the distance between the mirrors will change by a tiny amount when a gravitational wave passes by the detector, which can register a change in the lengths of the arms smaller than one-ten-thousandth the diameter of a proton (10-19 m). And one of the great challenges is to isolate real astronomical signals from sources of noise that could mimic – or simply drown out – the signal.

Advanced LIGO is a major upgrade that has increased the sensitivity of the instruments, increasing laser power, reducing noise and enabling them to probe a larger volume of the Universe.

Quite a big bang

Scientists estimate that the black holes detected on September 14, 2015 were about 29 and 36 times, respectively, the mass of the Sun. When they collided, they converted about the equivalent of three times the mass of the Sun into gravitational waves within a fraction of a second – with a peak power output of about 50 times that of the whole visible Universe

Black holes: the inside story

According to general relativity, two black holes orbiting each other lose energy by emitting gravitational waves, causing them to gradually come closer together over billions of years – then much more quickly in the final few minutes. During the final fraction of a second, the two black holes collide at nearly half the speed of light and form a single, bigger black hole, converting a significant proportion of their combined mass into energy – the effect observed by LIGO.